Aluminum and an alloy in which aluminum has added to it an impurity such as neodymium, silicon, or copper are inexpensive and have low resistance, and are therefore used as gate, source, and drain electrode materials for a liquid crystal display.
However, since aluminum and the aluminum alloy have somewhat poor adhesion to a glass substrate base, they are used as an electrode material in the form of a laminate film in which a molybdenum or molybdenum alloy film is used beneath the aluminum or aluminum alloy, and one-time etching is carried out by means of an etching solution employing phosphoric acid, etc.
In recent years, the price of molybdenum or a molybdenum alloy has been increasing steeply, and titanium or a titanium alloy has therefore received attention. However, it is impossible to carry out etching of titanium or a titanium alloy by means of the etching solution employing phosphoric acid, etc. that is used in aluminum/molybdenum laminate film etching.
It is known that when etching titanium or a titanium alloy in a process for fabricating a semiconductor device, a hydrofluoric acid-based etching solution is normally used (e.g. Patent Document 1). Furthermore, it is also known that a metal thin film containing titanium as a main component can be etched using an ammonia+hydrogen peroxide-based etching solution (e.g. Patent Document 2). Moreover, when etching a titanium/aluminum/titanium laminate film, an etching solution formed from a fluorine compound and an oxidizing agent has been reported (Patent Document 3).
However, when a hydrofluoric acid-based etching solution is used for etching of an aluminum/titanium laminate film, since the amount of the upper aluminum layer etched greatly exceeds the amount of the lower titanium layer etched, side etching increases, and there is the problem that wiring cannot be formed exactly as in a resist pattern. This also applies to a case in which an etching solution consisted of a fluorine compound and an oxidizing agent is used for etching of an aluminum/titanium laminate film; when the uppermost layer film is aluminum the susceptibility to a liquid reagent is higher than in the case when the uppermost layer film is titanium, and it becomes difficult to carry out a high precision etching treatment. With regard to the ammonia+hydrogen peroxide-based etching solution, since the etching rate for aluminum or titanium is very low and the stability of hydrogen peroxide is decreased when solution is basic, thus shortening the solution life, it cannot be said to be practical.
As described above, means suitable for one-time etching of a metal laminate film in which a layer formed from aluminum or an aluminum alloy as an upper part and a layer formed from titanium or a titanium alloy as a lower part are laminated has not yet been developed.